This invention relates generally to passive, organic light emitting device displays.
Organic light emitting device (OLED) displays offer the potential for bright, three-color displays for various applications. OLEDs emit light when current flows through the OLED material.
Two driving schemes are used to drive OLED displays. In an active driving scheme, transistors, such as thin film transistors, may be used to individually control the brightness of each pixel. In this scheme, a single pixel short results in a dark spot in a display that may be unnoticeable by human eyes. Due to the requirement of the OLED, crystalline silicon or high temperature polysilicon thin film transistors are usually used as controlling devices on the back plane of the display. These thin film transistors may add a relatively substantial cost to the display.
Work is ongoing to improve the performance of low temperature polysilicon and amorphous silicon based transistors. However, a cost-effective method for OLED displays is still not widely available.
Passively driven displays, on the other hand, provide a simple and cost-effective solution for small area OLED displays. With the development of tiling techniques, large area, cost-effective OLED displays become a possibility. With large area OLED displays, OLED pixels within each tile may be passively driven.
Since OLED devices are very thin (on the order of several hundred Angstroms), pixel shorts may occur. With local, passive matrix schemes, one shorted OLED pixel can take out an entire row and/or column that is associated with the short pixel.
Generally, in passive displays, a line is driven one at a time. In one example, an entire row may be activated at one time and then each of the columns selectively activated to light pixels along the row. Thus, if one pixel goes bad, the optical characteristics of the entire row may be adversely affected.
Referring to FIGS. 7 and 8, a pixel may be made up of a metal row 14 that extends transversely to a transparent conductive column 12. Between the row 14 and column 12 is the OLED 16. As shown in FIG. 8, the OLED is sandwiched between transverse rows 14 and columns 12. Light is emitted through the column 12 which is transparent. Once the OLED pixel is shorted, the row 14 and the column 12 that connect to the pixel are both shorted.
Thus, there is a need for a way to reduce the adverse consequences of pixel shorting in passive OLED displays.